This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Mercury contamination is an extremely important environmental problem, and it is particularly hazardous in sediments where the highly toxic methyl mercury (MeHg) concentrates. Attempts to clean up mercury-contaminated sediments by either physical or chemical remediation have been hampered by the fact that these procedures are expensive and environmentally destructive. As a result, there has been a great deal of focus to develop a bioremediation system to deal with mercury contamination. Bacteria isolated from organic mercury-contaminated sites have developed a system of two enzymes (MerA and MerB) that allows them to efficiently convert both ionic and organic mercury compounds to the less toxic elemental mercury. Many of the currently used bioremediation systems attempt to take advantage of the unique properties of MerA and MerB to clean up mercury contamination. Our objective is to gain a detailed structural and mechanistic understanding of how MerB are able to degrade organomercurial compounds to ionic mercury. We propose as a first step to collect highest resolution data on MerB crystals grown under a variety of screening conditions. In a second series of experiments, we will soak into MerB crystals appropriate organic mercurial agents to identify the mercury binding site(s) and accompanying structural changes if any. The avidity of MerB for organomercurials is substantial and will allow us to work at low concentrations (<10mM). In case of good isomorphism with native crystals, the organomercurial derivative data collected at a single wavelength will be used as a heavy atom to solve the MerB structure based on isomorphous and anomalous differences. In case of substantial non-isomorphism, we plan to collect MAD data using the anomalous mercury signal and solve the MerB structure bound with the organomercurial. Our objective being a detailed description of the mercury binding site on MerB.